lmb: Fix LMB_MEMORY_REGIONS flag usage

This patch is fixing a broken boot observed on stm32mp157c-dk2 board.

IS_ENABLED macro should be used to check if a compilation flag is set
to "y" or "m".
LMB_MEMORY_REGIONS is set to a numerical value, IS_ENABLED macro is not
suitable in this case.

Fixes: 7c1860fc ("lmb: Fix lmb property's defination under struct lmb")
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Acked-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Tom Rini <trini@konsulko.com>